With rapid development of digital printing technology such as thermal ink-jet printing, traditional printing papers face great challenges when used as the ink-jet print media. In addition to good image quality and low cost, print media today are generally expected to be able to dry quickly and to avoid image strike-through, especially when a sheet is printed on both sides.
The image quality, including ink optical density (OD), color gamut, and the ink drying time, which result from ink-jet printing greatly depend on the interaction of the ink and media, and especially on the ink-absorption capability of media. The ability of the paper stock to absorb aqueous solvents and the speed at which it absorbs the solvent are major considerations in the manufacturing of media for receiving ink-jet inks. For example, excessive absorption will bring the colorant into the bulk area of the base, resulting in low black and color optical density and low color gamut. This tends to create a “washed out” image. Poor absorption, on the other hand, creates a situation in which the ink bleeds and smears readily. Drying time of the ink is also increased when the paper stock has poor absorption characteristics.
Absorption of aqueous solvents by a paper stock is mainly controlled by the sizing processing of the paper stock during manufacture. This includes the processes of internal and surface sizing. The desired absorption properties of the paper stock for absorbing aqueous solvents are customarily controlled by balancing internal sizing and surface sizing.
To improve the quality of paper media printed with pigmented inks, a recently developed technology is now in use in the paper manufacturing process. This recent technology includes adding a divalent metal salt (e.g., calcium chloride), into the surface sizing composition during paper surface finishing processing. During subsequent use of the treated paper stock, when ink drops containing a pigment colorant are ejected onto the paper containing the metallic salt, the pigment “crashes out” of the pigment suspension solution. The metallic salt causes the pigmented colorant to separate from the aqueous solvent. In the case of calcium chloride salt treatment, calcium cations interact with anionic charged pigmented colorants and act as a colorant fixing agent. The salt causes the colorants to stay on the outermost surface of the print medium. In this way, the optical density of the image is increased and the ink drying time is reduced. In order to obtain satisfactory performance, a minimum amount of salt greater than 6-8 kg (salt)/T (paper) is customarily needed. However, such a high loading of chloride-containing compounds promotes drastic corrosion of the paper milling equipment used to produce the print media, and significantly reduces the life span of the salt-contacting parts of the paper manufacturing equipment, such as sizing rolls for instance.
Another drawback commonly associated with the use of calcium chloride salt arises from its exothermic dissolution in water. A significant amount of heat is produced when large batches of calcium chloride salt solution are prepared, as is customary in commercial paper manufacturing processes. Solution temperatures can easily reach 90-100° C. or more. Still another downside of using chloride salts in the manufacture of print media is that the associated chloride-containing vapors may pose health and safety issues for the operators. There is continuing interest in the development of ways to produce ink-jet print media that perform well with pigmented inks.